Selective Photocatalytic Synthesis of Haloanilines from Halonitrobenzenes over Multifunctional AuPt/Monolayer Titanate Nanosheet

Article abstract of DOI:10.1021/acscatal.8b02662

Bimetallic alloy AuPt nanoclusters supported on monolayer H_1.07Ti_1.73O₄·H₂O nanosheets (AuPt/TN) jointly complete a rapid catalytic reaction toward hydrogenation of halonitrobenzene to haloaniline in methanol under ambient conditions using HCOONH₄ as a hydrogen source. Especially, AuPt/TN with a Au/Pt molar ratio of 1:2 exhibits the high catalytic conversion efficiency for halonitrobenzene (>99%) with a high selectivity of haloaniline (>99%). In situ FTIR spectra suggest that the TN affords surface Br?nsted acid sites to chemisorb and activate the halonitrobenzene molecules via the surface hydrogen bond coordination. In situ ESR experiments indicate that HCOONH₄ would be decomposed to H⁺ and a ^?CO₂^- radical by photogenerated holes, serving as the hydrogen source and reducing species for the reduction of the -NO₂ group, respectively. Experimental results reveal that atom Pt in alloy is responsible for the hydrogenation, while Au represses the dehalogenation of haloanilines. Finally, a possible synergetic mechanism is discussed. This work highlights that the multifunctional AuPt/TN catalyst with multiple active sites exerts the respective functions to cooperatively catalyze organic transformations toward desired target products.

Full text of DOI:10.1021/acscatal.8b02662

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Article
Selective Photocatalytic Synthesis of Haloanilines from Halonitrobenzenes
over Multifunctional AuPt/Monolayer Titanate Nanosheet
Yujie Song, Huan Wang, Zhitong Wang, Binbin Guo, Kaiqiang Jing, Yanjun Li, and Ling Wu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02662 • Publication Date (Web): 11 Sep 2018
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ACS Catalysis
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Selective Photocatalytic Synthesis of Haloanilines from Halonitro-
benzenes over Multifunctional AuPt/Monolayer Titanate Nanosheet
Yujie Song, a Huan Wang, a Zhitong Wang, a Binbin Guo, a, b Kaiqiang, Jing, a Yanjun Li, c Ling
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Wu a, b*
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a. State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350116, P.
R. China.
b. State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese
Academy of Science, Fuzhou, Fujian 350002, P. R. China.
c. Shanghai Institute of Measurement and Testing, 1500 Zhangheng Road, Shanghai, 201203, P. R. China.
ABSTRACT: Bimetallic alloy AuPt nanoclusters supported on monolayer H1.07Ti1.73O4∙H2O nanosheets (AuPt/TN) jointly
complete a rapid catalytic reaction towards hydrogenation of halonitrobenzene to haloaniline in methanol under ambient
conditions using HCOONH4 as a hydrogen source. Especially, AuPt/TN with the Au/Pt molar ratio of 1:2 exhibits the high
catalytic conversion efficiency for halonitrobenzene (>99%) with a high selectivity of haloaniline (>99%). In-situ FTIR
spectra suggest that the TN affords surface Brønsted acid sites to chemisorb and activate the halonitrobenzene molecules
via the surface hydrogen bond coordination. In-situ ESR experiments indicate that HCOONH4 would be decomposed to
-
H+ and ∙CO2 radical by photogenerated holes, serving as the hydrogen source and reducing species for the reduction of -
NO2 group, respectively. Experiment results reveal that atom Pt in alloy is responsible for the hydrogenation, while Au
represses the dehalogenation of haloanilines. Finally, a possible synergetic mechanism is discussed. This work highlights
the multifunctional AuPt/TN catalyst with multiple active sites exerts the respective functions to cooperatively catalyze
organic transformations toward desired target products.
KEYWORDS: AuPt Nanoclusters, Monolayer titanate nanosheet, Photocatalytic organic transformation, Halonitroben-
zene, Haloanilines.
Au, Pt and Pd are usually used as a co-catalyst to activate
and dissociate H2 for hydrogenation of halonitroben-
1. INTRODUCTION
Selective hydrogenation of nitroaromatics to anilines
has always been a research hotspot because anilines are
indispensable precursors or intermediates in chemical
industries, especially, haloanilines. As the key interme-
diates for the production of pigments, pharmaceuticals,
pesticides and dyes, haloanilines are commercially pro-
zenes. However, they suffer from the issues that indis-
criminate hydrogenation of halogen (-X) and -NO2 group
results in the dehalogenation of haloaniline to aniline
under the pressure of H2. How to rationally fabricate and
construct appropriate photocatalysts for selective hydro-
genation of –NO2 would be great challenge to achieve the
highly efficient conversion of halonitrobenzenes to
haloanilines.
1-3
duced from the reduction of halonitrobenzenes using the
4-6
reductive metal Fe or Sn in HCl solution.
However,
many co-products, such as aniline intermediate and near-
ly equimolar amounts of metal salts, are produced in the-
se processes. Recently, heterogeneous catalytic processes
are developed to synthesize haloanilines via the direct
Two dimensional (2D) ultrathin structure materials ex-
hibit various distinctive advantages different from their
corresponding bulk materials. 2D structure has a huge
specific surface areas and unique surface states, such as
highly exposed specific crystal facets, abundant surface
unsaturated metal sites and dangling bonds. 24-27 Ultrathin
2D nanosheets have been widely reported as efficient het-
erogeneous catalysts, mostly based on their Lewis acid
sites supplied by surface unsaturated metal sites. 28-30 The-
se surface sites would efficiently chemisorb relevant mol-
ecules. Moreover, 2D nanosheets allow them to support
the highly dispersed metal nanoclusters for assembling
into a multifunctional photocatalyst. The surface charac-
ters on the nanosheets would enhance the interface inter-
actions with these metal nanoparticles (NPs). Very re-
hydrogenation of halonitrobenzenes with H2 as hydrogen
7-12
source.
A challenging topic is desired to improve the
selectivity of haloanilines because the weak carbon-
halogen (C-X) bond is easy to be dehalogenated forming
aniline. Therefore, an alternative photocatalytic process
may be potential to achieve the task.
A variety of photocatalysts are used for hydrogenation
of nitroaromatics to corresponding amines under mild
13-17
18-21
22
conditions, such as TiO2-based,
CdS,
HNb3O8,
and so on. For the photocatalytic synthesis of haloan-
23
ilines, only noble metals/TiO2 as catalysts are reported.
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cently, NPs/2D nanosheets, such as Au/TiS2, 31 Au/C3N4, 32
added. The suspensions were irradiated under a Xenon
lamp for 4h (PLS-SXE300D, 1000 mw/cm2, Beijing Perfect
light Co. Ltd.) with stirring in N2 atmosphere. The final
products were washed and collected, labelled as Pt/TN,
Au2Pt1/TN, Au4Pt1/TN, Au6Pt1/TN, Au/TN, Au1Pt1/TN,
Au1Pt2/TN, Au1Pt4/TN and Au1Pt6/TN respectively. For
33
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Pd/C3N4, 35, Pd/H1.07Ti1.73O4,
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Pt/C3N4,
Pd/TiO2,
Pt/TiO2,
have been reported in photocatalysis. Com-
37
pared with monometallic NPs, bimetallic alloy NPs with
bifunctional catalytic sites usually exhibit high catalytic
activity and selectivity due to the synergy of bimetallic
alloy nanoparticles. 38-40 Additionally, their electronic con-
figuration would be redistributed via tuning the compo-
nent of an alloy, governing the catalytic performance.
Thus, based on an idea of the cooperation between bime-
tallic alloy and ultrathin 2D nanosheet, a composite pho-
tocatalyst with multifunctional active sites may be de-
signed for the selective hydrogenation of halonitroben-
zenes to haloanilines.
comparison,
the
Au1Pt2/layered
H1.07Ti1.73O4∙H2O
(Au1Pt2/LT) and Au1Pt2/TiO2 were prepared via the same
method.
9
2.3 Photocatalytic reaction for hydrogenation of
halonitrobenzenes to haloanilines. 20 mg of catalyst,
0.5 mmol halonitrobenzenes and 3.5 mmol HCOONH4
were placed in a Pyrex reactor with a jacket under vacu-
um condition to remove oxygen. Reaction temperatures
were controlled via a circulating condensate water. Meth-
anol of 4 mL with oxygen eliminated was transferred to
the reactor. The mixed suspension was firstly stirred un-
der darkness for 20 min to promote fully dispersion of
catalysts and dissolution of reactants. The suspension was
then irradiated under a Xe-arc lamp of 300 W (PLS-
SXE300D, 1000 mw/cm2, Beijing Perfect light Co. Ltd.).
The IR-cut filter was also used to remove the wavelengths
that are longer than 800 nm. After definite reaction time,
the catalyst was removed from the mixed suspension via
centrifugation. The supernatant with final products was
determined by an Aglient GC 6890 (FID detector) and a
mass spectrometer of HP-5973.
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Herein, the bimetallic AuPt alloy nanoclusters support-
ed on the monolayer H1.07Ti1.73O4∙H2O nanosheets
(AuPt/TN) are prepared via an in-situ photoreduction
method to photocatalyze the selective hydrogenation of
halonitrobenzenes to haloanilines. On the basis of the
surface hydroxyl (-OH, Brønsted acid sites) in
H1.07Ti1.73O4∙H2O, Au and Pt (-OH-Au-Pt), the bimetallic
AuPt/H1.07Ti1.73O4∙H2O bearing three active sites as a mul-
tifunctional photocatalyst has cooperatively catalyzed the
selective hydrogenation of halonitrobenzenes to haloan-
ilines in the presence of HCOONH4 at room temperature
and O2 free condition. In-situ FTIR spectra are carried out
to fully reveal the surface adsorption behaviors of relevant
molecules on hydroxyl (-OH) in TN. Controlled experi-
ments are conducted to elucidate the influence of Au, Pt
and supports on activity and selectivity. Furthermore, the
interface electrons transfer in bimetallic alloy nanoclus-
ters are also deeply studied by XAFS analysis. Moreover,
roles of HCOONH4 are also examined by the results of
experiments and characterization in detail. Combining
with the ESR technique, we propose a possible three ac-
tive sites cooperatively catalytic mechanism for the highly
efficient photocatalytic hydrogenation of halonitroben-
zenes to haloanilines.
3. RESULTS AND DISCUSSION
2. EXPERIMENTAL SECTION
2.1 Synthesis for H1.07Ti1.73O4∙H2O nanosheets (TN).
The TN was prepared via an exfoliation process by inter-
calating the layered H1.07Ti1.73O4∙H2O (LT) precursor using
the TBAOH (40 %) as intercalation agent.
mole ratio for LT and TBAOH is about 1: 1.2. The solution
was stirred continuously for more than 7 days at room
temperature. The resulting white colloidal solution was
flocculated by adding 1 M HCl solution to obtain the solid
product of TN. Then, the obtained sample was washed
and dried at 60℃.
36, 45, 55, 56
The
Fig.1. XRD patters of the prepared bimetallic AuPt/TN with
several of mole ratios.
Bimetallic AuPt/H1.07Ti1.73O4∙H2O nanosheets (AuPt/TN)
with different mole ratio were prepared via an in-situ
photo-reduction method. For the determination of bulk
phase features, as-obtained samples were characterized
by XRD. Fig. 1 shows the XRD patterns for TN loaded by
AuPt alloy (Au1Pt6/TN, Au1Pt4/TN Au1Pt2/TN, Au/TN,
Pt/TN, Au1Pt1/TN, Au2Pt1/TN, Au4Pt1/TN and Au6Pt1/TN).
The characteristic peak around 38.4° is assigned to the
2.2 Preparation for metal nanoclusters
1.07Ti1.73O4∙H2O nanosheets (M/TN). 1% wt of AuPt
alloy supported on the TN was prepared via a photo-
/
H
36
deposition method.
The aqueous solutions of
41
AuPt (111) crystal face. Compared to that of Pt (111, 2θ:
HAuCl4∙4H2O (10 mg/mL) and H2PtCl6∙6H2O (10 mg/mL)
were mixed with the Au:Pt mole ratio of 0:1, 2:1, 4:1, 6:1,
1:0, 1:1, 1:2, 1:4 and 1:6, respectively. The mixed solution
was injected into the colloidal solution of TN. Then, 5 mL
of methanol, as the hole sacrificial agent, was drop-wise
39.1°), the peak shifts to a lower 2θ angle indexed to the
higher d space (d (111) = 0.229 nm). This d space is larger
than that of Pt (d (111) = 0.222 nm) and smaller than that
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of Au (d (111) = 0.234 nm), indicating the formation of
AuPt alloy.
Element-specific X-ray absorption near edge structure
spectroscopy (XANES) is a power technique to reveal the
electronic structures of AuPt alloy. With regard to the L3-
edge XANES of Au and Pt, electronic transition takes
place from 2p3/2 to 5d5/2, and the peak intensity of white
line (peak of absorption edge) increases with the unoccu-
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TEM images of the prepared Au1Pt2/TN are shown in
Fig. 2. It is observed that the metal nanoclusters are high-
ly dispersed on the ultrathin H1.07Ti1.73O4∙H2O nanosheets
(Fig. 2a). The nanoclusters show an average size of about
3nm (inserted in Fig. 2a). The high-resolution image con-
centrated on a single metal nanocluster shows the clear
lattice fringes with about 0.229 nm spacing (Fig. 2b),
which would be assigned to the fcc-type (face-centered
cubic) AuPt (111) planes with that of 0.226 nm for fcc-type
Pt (111) and 0.235 nm for fcc-type Au (111). 41 Moreover, the
lattice fringes with the distance of 0.218 nm would be at-
tributed to the new planes in bimetallic AuPt, because
lattice fringe spaces of 0.203 and 0.226 nm are corre-
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pied degree of 5d5/2. The bulk average electronic states
of Au and Pt in AuPt nanoclusters were further investi-
gated by XANES via the comparisons in intensity of L3
absorption edges (white line) from Pt and Au. As shown
in Fig. 3a, single Au metal shows no intense white line for
L3 edge due to a filled d10 orbital. However, the L3-edge
spectra of Au in AuPt alloy shows a higher white line peak
intensity than the Au foil, suggesting the electrons loss in
d character of Au. Conversely, the Pt L3 edge of white line
peak in the prepared sample is weaker than that in the
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sponding to Au (200) and Pt (111), respectively. A cub
single Pt foil (Figure 3b), arising from the gain of d-
model of single AuPt nanocrystal is schematically repro-
duced in Fig. 2b (inserted). These results suggest the suc-
cessful formation of intermetallic nanoclusters on the TN.
Furthermore, EDS further confirms the existence of Au,
Pt and Ti (Supporting Information, Fig. S1).
44
charges at Pt atom. According to the literature,
elec-
trons transfer would take place from a metal of more oc-
cupied valence band to another metal in an alloy for the
stability of bimetallic bonds. Therefore, the d charges
depletion in Au atom and the d-charge gain in Pt atom
support the fact that Au-Pt bond exists in AuPt alloy,
which coincided with the results of TEM, XRD and XPS.
Fig. 2 (a) TEM images of the prepared Au1Pt2/TN (Magnified
image has been inserted). (b) HRTEM image of AuPt alloy
nanoclusters (inserted is the AuPt cub model).
Fig. 3 XANES spectra of as prepared Au1Pt2/TN. (a) L3-edge
of Au and foil reference, (b) L3-edge of Pt and Pt foil refer-
ence.
To reveal surface chemical states of the prepared AuPt
alloy, XPS (X-ray photoelectron spectroscopy) analysis
was conducted. Full XPS spectra for Pt/TN, Au/TN and
Au1Pt2/TN are shown in Fig. S2a. Elements of C, Ti, Au, Pt
and O can be observed clearly, with their characteristic
peaks at the binding energy values of 73.7 (Pt 4f), 83 (Au
4f), 285 (C 1s), 459 (Ti 2p) and 530 eV (O 1s) respectively.
Fig. S1b shows the high-resolution for Au of Au/TN and
Au1Pt2/TN. For Au1Pt2/TN, the characteristic peaks with
binding energy values of 83.9 and 87.6 eV are attributed
to Au 4f7/2 and Au 4f5/2 respectively, indicating the metal-
41-43
lic Au0.
Furthermore, these peaks shift to the higher
binding energy values, compared with those for Au/TN
(83.6 and 87.3eV). These results would be attributed to
the redistribution of electrons between Au and Pt in AuPt
alloy. Moreover, the binding energy values of Pt 4f exhibit
shift from 70.9 (4f7/2) 74.2eV (4f3/2) for Pt/TN to 70.5 and
73.8eV for Au1Pt2/TN, indicating that the electron cloud
distribution moves from Au atom to Pt atom in the AuPt
alloy nanoclusters. These results support the interaction
of Au and Pt, in accordance with the results of XRD and
TEM.
Fig. 4 Photocatalytic performance of prepared samples. Reac-
tion conditions: catalyst, 20 mg, 4-chloronitrobenzene, 0.5
mmol, HCOONH4, 3.5 mmol, solvent, 4 mL methanol, at-
mosphere, 1 atm N2, 298 K. Reaction temperature was con-
trolled via a jacket connected to a circulating cold water
source. Xenon lamp with an IR-cut filter to remove all wave-
lengths longer than 800 nm.
The photocatalytic performances of prepared samples
were examined for the hydrogenation of halonitroben-
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zene in methanol solvent at N2 atmosphere. Fig.4 shows that photocatalysis is mainly responsible for the perfor-
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the photocatalytic activities of the prepared samples. It is
obvious that Pt/TN, Au1Pt2/TN, Au1Pt4/TN and Au1Pt2/TN
give the high conversions of 4-chloronitrobenze (＞99%)
after 4h, whereas, Au/TN, Au1Pt2/TN, Au2Pt1/TN,
Au4Pt1/TN and Au6Pt1/TN exhibit the high selectivity for
the 4-chloroaniline. In particular, Au1Pt2/TN shows both
the excellent conversion of 4-chloronitrobenzene and
selectivity to 4-chloroaniline (>99%). These results sug-
gest that Pt would contribute to the conversion of 4-
chlorinenitrobenzene, and Au is responsible for the selec-
tivity of the desire product. The turnover frequency (TOF)
and turnover number (TON) are also calculated by the
moles of converted 4-chloronitrobenzene per mole of
AuPt in catalyst per hour and produced 4-chloroaniline
per mole of AuPt, respectively. Thus, the TOF for
Au1Pt2/TN is 122.3 h-1, and TON is calculated as 489.2 in 4
h.
mances. Furthermore, the layered H1.07Ti1.73O4∙H2O (LT)
and TiO2 (Anatase) were also used as the supports to re-
veal the roles of TN. It is obvious that Au1Pt2/TiO2 and
Au1Pt2/LT show the disappointing catalytic performances
under the identical conditions. These results indicate that
the selective hydrogenation of 4-chloronitrobenzene is
driven by AuPt alloy with the assistance of TN.
As a photoactive semiconductor material, the photoab-
sorption property would influence its catalytic perfor-
mance. The TN only adsorb UV-light with the wavelength
less than 360 nm (λ≤360 nm). The deposition of AuPt
alloy would promotes TN to adsorb more visible light
based on the surface plasma resonance of metal nanoclus-
ters (Fig. S3). Thus, we further examine the light effects of
Au1Pt2/TN. As Fig. S4 shows that, under the visible light
irradiation (420 nm≤λ≤800nm), 27% conversion of 4-
chloronitrobenzene with 98% selectivity of 4-
chloroaniline is achieved in the same conditions. Consid-
ering that TN would not be excited by visible light, AuPt
nanoclusters would be responsible for photocatalysis, due
to their hot electron actions. This is why the Au1Pt2/TN
also show a thermal catalytic activity. Remarkably, a con-
siderable accelerating effect is observed under a light
(350nm≤λ≤800nm) irradiation. The enhanced photo-
catalytic activity would be attributed to the excitation of
TN. These results revealed that TN would act as a photo-
catalyst to supply photogenerated electrons and holes.
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As shown in Fig. 5, the yield variations of 4-
chloroaniline with time were also recorded over Pt/TN
and Au1Pt2/TN. For Pt/TN (Fig. 5a), it has been observed
that the yield of 4-chloroaniline is firstly increased from 0
to 42% within 2 h and then decreased to 20% as the reac-
tion time prolonged. However, the yield of byproduct
aniline is continuously increased from 0 to 80% within
reaction time. A small amount of nitrobenzene is also
detected
due
to
the
dehalogenation
of
4-
chloronitrobenzene. These results indicate that Pt/TN has
no selectivity for the hydrogenation of -Cl and -NO2. Fig.
5b shows that the yield of 4-chloroaniline is constantly
increased with the reaction time prolonged. The byprod-
ucts aniline and nitrobenzene are not detected in the
whole process. All of these results demonstrate that the
introduction of Au in Pt/TN would inhibit the dechlorina-
tion of 4-chloroaniline and 4-chloronitrobenzene. Com-
bined with the XANES spectra, it can be deduced that the
electrons transfer from Au to Pt atom in AuPt alloy would
dominate the selectivity of 4-chloroaniline via the recom-
bination of electronic structures.
Fig. 6 Yield of 4-chloroaniline with the various content of
HCOONH4. Au1Pt2/TN 20 mg, solvent, methanol 4 mL, 4-
chloronitrobenzene, 0.5 mmol, atmosphere, N2 1atm, λ≤
800 nm, 298 K, reaction time 4 h.
Fig. 5 The yield variations of 4-chloroaniline with time over
Pt/TN (a) and Au1Pt2/TN (b). Identical reaction conditions as
described in Fig. 4.
To better elucidate the roles of HCOONH4, Control ex-
periments are carried out (Fig.6). Negligible 4-
chloroaniline is detected over Au1Pt2/TN in the absence of
HCOONH4. The yield of the 4-chloroaniline increases
with the increase of HCOONH4 content. When 3.5 mmol
of HCOONH4 is used, the yield of 4-chloroaniline reaches
the highest value with the yield of 99%. With the content
of HCOONH4 further increase, the yield of desired prod-
uct then decreases. Meanwhile, a large quantity of aniline
is detected, which would be attributed to over hydrogena-
tion of 4-chloroaniline. It should be noted that the gener-
ation rate of 4-chloroaniline firstly increases and then
decreases as the increased content of HCOONH4, indicat-
Moreover, series of control experiments were conduct-
ed to better understand this reaction. The results are
listed in Table S1. No desired product is detected with
absence of a catalyst or alone TN (entries 1-2), suggesting
that AuPt alloy is indispensable for this reaction. It is ob-
vious that 14% of 4-chloronitrobenzene is catalyzed to 4-
chloroaniline under the darkness at 298K. Even elevating
reaction temperature to 353K, the conversion of 4-
chloronitrobenzene only increases to 31 % with 91 % of
selectivity for 4-chloroaniline (entries 3-4), suggesting
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The stability of a catalyst is extremely important in ap-
ing that the induced process of HCOONH4 may suffer
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from other competitive reactions. It can be inferred that
the HCOONH4 may act as the hydrogen source.
plication. We also tested the cyclical stability of
Au1Pt2/TN in this reaction. To avoid the interference of
excessive catalyst, the halved Au1Pt2/TN (10 mg) was used
to examine the catalytic stability. As shown in Fig. S5,
Au1Pt2/TN maintains the stable catalytic conversion for
the 4-chloronitrobenzene with the high selectivity of 4-
chloroaniline even after the five recycles. After the reac-
tion, the filtrate was analyzed by inductively coupled
plasma atomic emission (ICP-AES). No Au or Pt was de-
tected, confirming the stability of the Au1Pt2/TN catalyst.
We also studied the influence of reaction atmosphere
on the induced process of HCOONH4. It is obvious that a
sharply decreased conversion and selectivity are observed
at air atmosphere, compared with that at Ar or N2 atmos-
phere (Fig. 7). These results demonstrate that the induced
HCOONH4 may produce some active intermediate spe-
cies, participating in the reaction under the production of
oxygen free atmosphere. Such catalytic process would be
explicitly explored in the later content.
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44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Selectively Photocatalytic Hydrogenation of
Halonitrobenzenes to Haloanilines over the bimetallic
Au1Pt2/TN.
Entry
-X
Time (h)
Conv. (%)
＞99
＞99
＞99
99
Sel. (%)
99
1
4-Cl
3-Cl
2-Cl
4-Br
3-Br
2-Br
4-I
4
3.7
4
2
3
4
5
6
7
8
9
99
96
4.6
4.2
4.5
6
98
99
90
99
87
99
96
3-I
6
99
94
Fig.7 The photocatalytic activities of Au1Pt2/TN for selective
hydrogenation of 4-chloronitrobenzene at different reaction
conditions. Au1Pt2/TN, 20 mg, methanol 4mL, 4-
chloronitrobenzene, 0.5 mmol, HCOONH4, 3.5 mmol,
N2/Ar/Air/O2 1atm, λ≤800 nm, 298 K, reaction time 4 h.
2-I
8
98
68
Reaction conditions: Au1Pt2/TN, 20 mg, HCOONH4, 3.5
mmol, halonitrobenzene, 0.5 mmol, methanol 4 mL, N2, 1
atm, 298 K, Xe-lamp with an IR-cut filter (λ≤800 nm).
To better understand the effect of solvents on catalytic
efficiency, water, toluene, acetonitrile and benzene were
used as the solvent for photocatalytic hydrogenation of 4-
chloronitrobenzene over Au1Pt2/TN at 298 K under N2
atmosphere. Results are listed in Table S2. In water sol-
vent, only 6.9% yield of 4-chloroaniline is obtained after
the reaction time of 4 h due to the small solubility of 4-
chloronitrobenzene. Moreover, in organic solvents, the
conversion of 4-chloronitrobenzene increases with the
increasing polar of the solvent (Acetonitrile > Toluene
>Benzene). However, all of them suffer from the low con-
version of 4-chloronitrobenzene and selectivity of 4-
chloroaniline. As shown in Table S2, only 21% conversion
of 4-chloronitrobenzene and 7% yield of 4-chloroaniline
are detected in acetonitrile solvent under the identical
conditions. Moreover, 11% and 6% conversion of 4-
chloronitrobenzene are obtained in toluene and benzene
solvent, respectively. However, the desired 4-
chloroaniline is undetected (Table S2, entries 3-4). The
insolubility of HCOONH4 in acetonitrile, toluene and
benzene solvents may also result in the deficiency of hy-
drogen source to restrain the hydrogenation of 4-
chloronitrobenzene. Compared with toluene, acetonitrile
and benzene, methanol is a more suitable solvent for this
reaction due to the better dissolution for 4-
chloronitrobenzene and HCOONH4.
Moreover, several halonitrobenzenes are expanded to
detect the general applicability of the catalyst. The pre-
pared Au1Pt2/TN was used in hydrogenation of a series of
halonitrobenzenes in the presence of HCOONH4. The
results are shown in Table 1. It is obvious that 2- and 3-
chloronitrobenzenes can be selectively catalyzed into cor-
responding chloroaniline (entries 2-3). Similar with -Cl,
nitrobenzene with the -Br and -I substituted are also hy-
drogenated to haloanilines with the high selectivity (en-
tries 4-9). It should be noted that the 2- and 3- substitut-
ed halonitrobenzenes are easier to be dehalogenated pro-
ducing anilines, due to the spatial effect of -X and -NO2. 11
It would be observed that the stronger the electron-
withdrawing ability for substituent -Cl, -Br, -I, the higher
45
the catalytic activities are.
These would be attributed
that the functional groups with electron-withdrawing
would weaken bond energy of N-O to promote its activa-
tion. Thus, the reaction rate follows the order of -I< -Br <
-Cl, which is similar to the previous studies. 11, 45
To further elucidate the active species, a simulated in-
situ electron paramagnetic resonance technic (in-situ
EPR) coupled with DMPO spin-trapping was carried out
at N2 atmosphere under light irradiation. Results are
shown in Fig. 8. It can be observed that a characteristic
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Page 6 of 11
(1)
-
+
-
peak with sextet of DMPO-∙CO2 is observed in presence
of HCOONH4 for Au1Pt2/TN, Pt/TN and Au/TN. No ap-
parent signals are examined in absence of HCOONH4 or
under darkness, suggesting that the formation of ∙CO2
derives from HCOONH4 with assistance of TN via a pho-
HCOONH4
NS + hⱱ
HCOO- + h+
AuPt + HCOO-
NH4 +HCO2
1
2
3
4
5
6
7
e- + h+
(2)
(3)
(4)
(5)
-
∙CO2 (-1.8V vs NHE) + H+
-
-
∙CO2 + AuPt-H
-
AuPt + H+ + e-
AuPt-H
to-induced process (Fig. 9). ∙CO2 radicals possess the
-
22, 46-47
-
strong reducibility (E0 (CO2/∙CO2 ) = -1.8 V),
which
6∙CO2 + 6AuPt-H + -NO2
would be fleetly consumed at air or O2 atmosphere. Thus,
the intensity of the ∙CO2 signal is decreased enormously
6 CO2 + 6AuPt + -NH2 +2 H2O
(6)
-
8
9
It has been reported that the properties of supports, in-
cluding surface acidity, basicity and unsaturated metal
atoms, would affect the catalytic conversion and selectivi-
ty by dominating the adsorption of reactant or product
molecules. In our previous studies, we also found that the
nanosheets with abundant surface Lewis acid sites and -
OH groups would efficiently adsorb relevant molecules
in air atmosphere (Fig. 9), resulting in a lower catalytic
efficiency (Fig. 7). The dehydrogenation of HCOONH4
and the reduction of nitro compounds process are shown
from Eqs. (1) to (6). HCOONH4 would dissolve in metha-
nol solution to produce HCOO- and H+ (Eqs. (1)). Under
the light irradiation, electrons and holes are generated in
10
11
12
13
14
15
16
17
18
19
20
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22
23
24
25
26
27
28
29
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31
32
33
34
35
36
37
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42
43
44
45
46
47
48
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53
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59
60
TN (Eqs. (2)). HCOO- is induced to produce H+ and ∙CO2
-
via forming surface coordination species, which signifi-
by photo-generated holes, because of the more positive
28, 36, 44, 53-54
22, 46-48
cantly promotes the catalytic efficiency.
To
valence band position (2.42 V vs. NHE) (Eqs. (3)).
deeply reveal the role of TN, we used the in-situ FTIR
technic to study the adsorption behaviors of 4-
chloronitrobenzene on the TN surface. As shown in Fig.
Meanwhile, the negative charges accumulated in AuPt
nanoclusters would also promote the dehydrogenation of
-
HCOO- to form ∙CO2 (Eqs. (4)). 2, 22 As a electrons sink,
10
A,
characteristic
peaks
assigned
to
4-
surface AuPt would reserve the photo-generated electrons
chloronitrobenzene are evidently observed after the ad-
sorption of 4-chloronitrobenzene on TN (Fig. 10 A (b)).
Even after the further evacuation, these peaks also re-
main, suggesting the strong interactions between relevant
molecules and TN (Fig. 10 A (c)). Moreover, the peaks at
the wavenumber of 1311 and 1528 cm-1 attribute to ν-NO2
vibration for 4-chloronitrobenzene adsorbed on TN.
Compared with those for free 4-chloronitrobenzene, these
peaks show a shift towards higher wavenumber, while
other characteristic peaks have no change (Fig. 10 A (d)).
These results further demonstrate that it is the -NO2 in-
stead of -Cl that coordinate with TN, and the interface
charge redistribution takes place via the coordination
species. Considering the abundant surface -OH groups
(Brønsted acid sites) in TN, it is rational to propose that
4-chloronitrobenzene would be adsorbed on TN via a Ti-
to improve separation of electron-hole pairs and promote
49, 50
the reduction of H+ (Eqs. (5)).
The unstable AuPt-H
would decompose to produce H for the hydrogenation of
–NO2. 51, 52 The reductive ∙CO2 and H released from AuPt
-
corporately complete the reduction of -NO2 group. Final-
ly, the -NH2 group is formed (Eqs. (6)). 17
45
O-H∙∙∙O=N or Ti-O-H∙∙∙O-N coordination. It is to say,
the available surface -OH groups should be responsible to
improving the adsorption efficiency. To confirm this de-
duction, the counterpart layered titanate (LT) with few
content of surface -OH groups was used for comparison.
Fig. 10 (B) shows that no obvious characteristic peaks at-
tributed to 4-chloronitrobenzene are observed after an
adsorption process (b), suggesting that a small amount of
relevant molecules would be chemisorbed on the surface
of LT. To further explore the influence of adsorbed nitro-
benzene on light absorption, UV-DRS spectra of single
TN and TN adsorbed by 4-chloronitrobenzene were ex-
amined. As shown in Fig. S6, the light adsorption of TN
shows a slight red shift to visible light region after the
adsorption of 4-chloronitrobenzene, suggesting that in-
terface charge transfer takes place between TN and 4-
chloronitrobenzene. The formed surface coordination Ti-
O-H∙∙∙O=N or Ti-O-H∙∙∙O-N species would greatly facili-
tate the activation of -N-O or -N=O bonds via interface
charges redistribution. Thus, the coordinated 4-
chloronitrobenzene is easier to be converted. Under visi-
ble light irradiation (420 nm≤λ≤800 nm), 27% conver-
Fig. 8 EPR spectra of Au/TN, Pt/TN and Au1Pt2/TN in a
methanol solvent with presence of DMPO at N2 atmosphere.
Fig. 9 EPR spectra of Au1Pt2/TN in methanol solvent with
presence of DMPO at different atmosphere.
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ACS Catalysis
NO2 or surface Au1Pt2 nanoclusters. The direct electrons
sion of 4-chloronitrobenzene with 98% selectivity of 4-
1
2
3
4
5
6
7
8
chloroaniline is also obtained (Fig. S4). Even though TN is
not excited by visible light, the activated –NO2 could be
hydrogenated into 4-chloroaniline by HCOONH4 with
assistance of Au1Pt2 nanoclusters, because the hot elec-
transfer take place from CB of TN to adsorbed –NO2 re-
sulting in the direct reduction of -NO2. Then, surface
Au1Pt2 nanoclusters induce the formation of active hydro-
gen via hot electrons for selective hydrogenation of re-
duced –NO2 to –NH2. Moreover, the photogenerated elec-
trons would also migrate to the surface Au1Pt2 nanoclus-
ters for the reduction of H+ into activated H to directly
hydrogenate the activated -NO2 groups. Because of the
electrons transfer from Au to Pt atom in AuPt alloy, the
generation rate of H would be efficiently regulated, de-
pressing the over hydrogenation of produced 4-
chloroaniline to aniline. Therefore, the cooperation of TN
and bimetallic synergy jointly governs the conversion of
4-chloronitrobenzene to 4-chloroanline with a highly
selectivity.
-
trons of Au1Pt2 would induce the generation of ∙CO2 from
HCOONH4 (Eqs. (4)). Furthermore, the deposition of
Au1Pt2 would promotes the light adsorption of TN (Fig.
S3), facilitating the catalytic process. Therefore, surface
coordination and activation of –NO2 on TN are one of the
crucial factors for high catalytic efficiency. The chemi-
sorption of reactant molecules on catalysts would lead to
significant reduction of kinetic barriers in the interface
reaction due to the interface charge redistribution. There-
fore, Au1Pt2/TN performed the superior catalytic activity
than Au1Pt2/LT and Au1Pt2/TiO2 (Table S1, entries 5-6). All
of these results support that TN not only serves as a sup-
port for AuPt alloy, but also provides abundant active
sites for the coordination and activation of 4-
chloronitrobenzene. TN with more open structure and
exposed surface acid sites could absorb more 4-
chloronitrobenzene molecules to greatly promote the
conversion efficiency.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 1. Proposed mechanism of photocatalytic
hydrogenated reduction of 4-chloronitronenzene
over the Au1Pt2/TN with presence of HCOONH4 under
darkness.
Fig.10. In-situ FTIR spectra of the TN (A) and LT (B) for 4-
chloronitrobenzene adsorption. (a) After degassing at 180 ℃
for 4 h. (b) Adsorption of 4-chloronitrobenzene for 30 min at
room temperature (physisorption+chemisorption). (c) Fur-
ther evacuation of excess probe molecules at 180 ℃ for 5 min
(chemisorption). (d) The FTIR spectrum of the 4-
chloronitrobenzene in CCl4 solution.
Based on the aforementioned experimental results and
discussion, we propose a possible mechanism for photo-
catalytic
chloroanline.
4-chloronitrobenzene
As shown in
reduction
Scheme
to
1,
4-
4-
chloronitrobenzene molecules would be adsorbed on the
surface -OH groups of TN via the hydrogen bonds form-
ing the Ti-O-H∙∙∙O=N or Ti-O-H∙∙∙O-N coordination, re-
sulting in the activation of -N-O and -N=O due to the
interface charges redistribution and transfer. HCOONH4
Scheme 2. Proposed mechanism of photocatalytic
hydrogenated reduction of 4-chloronitronenzene
over the Au1Pt2/TN with presence of HCOONH4 under
simulated sunlight irradiation.
would exist in the form of HCOO- and NH4 in polar
+
4. CONCLUSION
methanol solution. Under the darkness or visible light
irradiation, Au1Pt2 nanoclusters would induce HCOONH4
In conclusion, a multifunctional Au1Pt2/TN photocata-
lyst with multiple active sites is designed to exert the re-
spective functions for efficiently catalytic selective con-
version of halonitrobenzenes to haloanilines. It is re-
vealed that the TN afford abundant surface Brønsted acid
sites to selectively chemisorb -NO2 group via hydrogen
bonds forming the Ti-O-H∙∙∙O=N or Ti-O-H∙∙∙O-N coor-
dination. The formed surface coordination species would
activate the –N-O and –N=O bonds due to the interface
-
to produce hydrogen and strongly reductive ∙CO2 for re-
22
duction and hydrogenation of the activated –NO2. Un-
der simulated sunlight irradiation, electron and hole pairs
are generated in excited TN (Scheme 2). The holes would
be consumed by HCOO- promoting the generation of
-
-
46-47
∙CO2 radicals (HCOO-+h+→H++∙CO2 )
to further re-
duce the activated -N-O and -N=O. Synchronously, the
generated electrons may directly transfer to adsorbed –
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Page 8 of 11
for a Highly Selective Visible-Light Photocatalyst for the Reduc-
charge redistribution. Moreover, TN is photoactived to
generate electrons and holes. HCOONH4 is induced by
tion of Nitroaromatics to Anilines. ACS Catal. 2016, 6, 1744-1753.
(3) Liu, Z.; Huang, Y.; Xiao Q.; Zhu H. Selective reduction of
nitroaromatics to azoxy compounds on supported Ag–Cu alloy
nanoparticles through visible light irradiation. Green Chem.
2016, 18, 817-825.
(4) Takasaki, M.; Motoyama, Y.; Higashi, K.; Yoon, S.; Mo-
chida, I.; Nagashima, H. Chemoselective hydrogenation of ni-
troarenes with carbon nanofiber-supported platinum and palla-
dium nanoparticles. Org. Lett. 2008, 10, 1601-1604.
1
2
3
4
5
6
7
8
-
holes to produce strong reductive ∙CO2 and H+. Mean-
while, generated electrons would directly transfer to the
adsorbed –NO2 and surface Au1Pt2 alloy nanoclusters for
the reduction of –NO2 and production of active H. It has
been clarified that electronic structure of Au1Pt2 alloy
would tune the generate rate of H due to the electrons
transfer from Au to Pt atom, inhibiting the dechlorination
of produced 4-chloroaniline and 4-chloronitrobenzene.
The cooperation of TN with abundant surface acid sites
and AuPt synergistic effect with tunable electron struc-
ture jointly improve the selectivity of 4-haloanilines with
the high conversion of 4-halonitrobenzene. This work
would provide a strategy for the rational design of multi-
functional catalysts and help to understand the photo-
catalytic organic transformation at a molecular level.
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(6) Hodgson, H. H. The Action of Sulphur on the Mono‐
Chloroanilines-A Further Contribution towards Elucidating the
Mechanism of Aromatic Thionation and the Structure of Sul-
phide Dyestuffs. J. Soc. Dyers Colour. 1926, 42, 76-80.
(7) Li, J.; Shi, X.; Bi, Y.; Wei, J.; Chen, Z. Pd Nanoparticles in
Ionic Liquid Brush: A Highly Active and Reusable Heterogene-
ous Catalytic Assembly for Solvent-Free or On-Water Hydro-
genation of Nitroarene under Mild Conditions. ACS Catal. 2011,
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catalytic properties over nanocomposite catalysts for selective
hydrogenation of halonitrobenzenes. J. Catal. 2008, 255, 335-342.
(9) Xu, D.; Hu, Z.; Li, W.; Luo, S.; Xu, Z. Hydrogenation in ion-
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ASSOCIATED CONTENT
Supporting Information is available free of charge on the ACS
Publications website at DOI: XXXXXX.
The detailed experiments and supplementary experimental
results.
AUTHOR INFORMATION
Corresponding Author
(10) Motoyama, Y.; Lee, Y.; Tsuji, K.; Yoon, S.; Mochida, I.; Na-
gashima, H. Platinum Nanoparticles Supported on Nitrogen-
doped Carbon Nanofibers as Efficient Poisoning Catalysts for the
Hydrogenation of Nitroarenes. ChemCatChem 2011, 3, 1578-1581.
(11) Iihama, S.; Furukawa, S.; Komatsu, T. Efficient catalytic
system for chemoselective hydrogenation of halonitrobenzene to
haloaniline using PtZn intermetallic compound. ACS Catal. 2016,
6, 742-746.
(12) Cardenas, L. F.; Keane, M. A. The development of gold
catalysts for use in hydrogenation reactions. J. Mater. Sci. 2013,
48, 543-564.
(13) Wu, W.; Wen, L.; Shen, L.; Liang, R.; Yuan, R.; Wu, L. A
new insight into the photocatalytic reduction of 4-nitroaniline to
p-phenylenediamine in the presence of alcohols. Appl. Catal. B:
Environ. 2013, 130-131, 163-167.
(14) Shiraishi, Y.; Togawa, Y.; Tsukamoto, D.; Tanaka, S.; Hirai,
T. Highly efficient and selective hydrogenation of nitroaromatics
on photoactivated rutile titanium dioxide. ACS Catal. 2012, 2,
2475-2481.
(15) Chen, P.; Khetan, A.; Yang, F.; Migunov, V.; Weide, P.;
Stürmer, S.; Guo, P.; Kahler, K.; Xia, W.; Mayer, J.; Pitsch, H.;
Simon, U.; Muhler, M. Experimental and Theoretical Under-
standing of Nitrogen-Doping-Induced Strong Metal-Support
Interactions in Pd/TiO2 Catalysts for Nitrobenzene Hydrogena-
tion. ACS Catal. 2017, 7, 1197-1206.
(16) Tsutsumi, K.; Uchikawa, F.; Sakai, K.; Tabata, K. Photoin-
duced Reduction of Nitroarenes Using a Transition-Metal-
Loaded Silicon Semiconductor under Visible Light Irradiation.
ACS Catal. 2016, 6, 4394-4398.
(17) Piccinini, P.; Minero, C.; Vincenti, M.; Pelizzetti, E. Photo-
catalytic interconversion of nitrogen-containing benzene deriva-
tives. J. Chem. Soc., Faraday Trans. 1997, 93, 1993-2000.
(18) Xiao, F.; Miao, J.; Liu, B. Layer-by-layer self-assembly of
CdS quantum dots/graphene nanosheets hybrid films for photo-
electrochemical and photocatalytic applications. J. Am. Chem.
Soc. 2014, 136, 1559-1569.
* Email: wuling@fzu.edu.cn (L. Wu).
Author Contributions
In this manuscript, Y. Song, H. Wang, Z. Wang and K. Jing
carried out the synthesis and characterizations of materials
and the photocatalysis experiments. B. Guo and Y. Li did the
XPS and TEM characterizations. L. Wu conceived the pro-
ject, supervised the research work and discussed the results.
All authors contributed to the paper writing. We thanks Hao
Zhang (Shanghai Institute of Applied Physics, Chinese Acad-
emy of Sciences) very much for his great assistance in exper-
iments of XANES.
Funding Sources
This work was supported by the National Natural Science
Foundation of China (21872032 and 51672048), the major
science and technology projects of Fujian Province
(2015YZ0001-1).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Thanks to the Shanghai Synchrotron Radiation Facility of
China for the XAFS spectra measurements at the BL14W1
beam line.
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